Knee implants are often used to replace knees that become disabled or cause a patient pain due to normal wear of the bone or due to degenerative disease. A total knee replacement involves replacement of the native bone with at least two components: a femoral component to replace the distal end of the femur, and a tibial component to replace the proximal end of the tibia. The femoral and tibial components are positioned and designed to mimic the native bone and provide an articulation interface that allows normal anatomic movement of the knee joint following implantation surgery.
A standard tibial implant includes two main portions—a tray and a stem. The proximal portion of the tibial implant is a tray that forms the articulation interface with a femoral component. Often, the tray holds a liner made of a compliant material, such as polyethylene, that provides a smooth surface for articulation with the femoral component. The distal portion of the tibial implant forms a stem that is designed to extend down into a tibia into which the component is implanted. In order to provide normal anatomic movement of the knee implant, the tibial implant must be held firmly in place to prevent the implant from moving down further into the bone and also from rotating in place within the bone.
In order to resist movement of the tibial implant relative to the bone, the implant must form a strong interface with the bone into which it is being implanted. During implantation, the interface between the implant and the bone is first formed by an initial fixation, for example with bone screws or cement, when the tibial component is first placed into the bone. The initial fixation is sometimes supplemented by later ingrowth of the surrounding bone into the surfaces of the implant if the implant includes textured ingrowth surfaces. This ingrowth can provide some resistance to rotation and subsidence of the tibial component down into the bone. However, the cancellous bone that grows into the implant surface is soft, spongy bone that is able to resist only small forces. A substantial force applied to the implant can break the interface of the bone and the ingrowth surface, leading to subsidence and rotation of the tibial tray.
In some current approaches, the stem of a tibial implant is placed into the cancellous bone of a patient's tibia, and the areas between the implanted component and the hard cortical bone are filled with cement. While this cement provides a connection between the implant and the stronger cortical bone, the cement is often not strong enough to provide the initial and long term fixation and resistance against subsidence and rotation that is needed to resist high forces that can occur during normal use of a total knee arthroplasty. As with cancellous bone ingrowth, the cement interface can be broken by these forces, and the implant function can be compromised by the effects of stress shielding and subsidence.